Multiple-input, multiple-output (MIMO) technology offers significant increases in data throughput and link range without additional bandwidth or increased transmit power, and has thus attracted attention in wireless communications. MIMO techniques spread the same total transmit power over multiple antennas to achieve an array gain that improves the spectral efficiency and/or diversity gain that improves the link reliability, e.g., reduced fading. MIMO is also planned to be used in Mobile radio telephone standards such as recent 3GPP and 3GPP2. In 3GPP, Long Term Evolution (LTE) standards utilize MIMO techniques.
In MIMO, multiple transmitters and receivers are used to transfer more data at the same time while taking advantage of multipath effects where transmitted information bounces off walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times. MIMO takes advantage of multipath to combine the information from multiple signals improving speed and data integrity.
A MIMO radio transceiver is use to support processing of multiple signals for simultaneous transmission via a plurality of antennas and to support receive processing of multiple signals detected by the plurality of antennas. Voltage controlled oscillators are used to generate a local oscillator signal that are used to generate RF transmit signals and to down-convert received RF signals to intermediate frequency signals for processing.
MIMO radio transceiver designs may derive a LO (local oscillator) frequency using a divider that is embedded inside the RF chain, e.g., in each of the chains there is a divider that creates the LO frequency (Local LO Gen). The relative phase of the LO signal generated by the local LO generation divider has effects on system performance, such as DC in the receive path, which need to be mitigated. In order for the MIMO radio communications system to work best and reduce the overhead of dealing with these changes, it is valuable for the local LO Gen dividers to create signals with a relative constant phase. Synchronizing the LO Gen phase between the MIMO chains is a valuable consideration for WiFi transceivers that support beam-forming.
There are a few ways to mitigate these effects. A oscillator generation circuit that drives the LO frequency from the central location may be used. The use of a oscillator generation circuit is not practical because of the numerous high frequency traces that need to be used, and the strict restrictions on their layout. A second method is to keep the local LO generation circuit in operation for very long periods. However, this method is wasteful in power because even when the local LO generation is not working with the chains, the non-operational chains still consume power. A third method involves sending a synchronization signal from the central part of the circuit to the local logs. This adds another high frequency line per chain, e.g., 3 lines for a 3×3 system, that needs to be simulated, and therefore adds unnecessary complexity to the system and chip layout.